The study of the inner workings of music. Includes things like scales, harmony, rhythm, and why it's fundamentally impossible to tune a piano perfectly.

Music is extremely mathematical, much to my own personal delight. Everything fits into a well-defined system, and can be described in simple, non-ambiguous terms. Until you study jazz, that is.

And yet there are always mysteries that linger. One of my favorites is this: how is it that two notes in the interval of an octave manage be different, and yet in some undiscernable, indescribable way, sound the same? Yes, I know that the wavelength of the lower pitch is exactly twice as long as the higher one, but that still doesn't explain the intangible way that they can sound the same and yet different.

Not to mention that once everything seems to be making sense, jazz comes along and breaks every single rule you ever learned. Set your cat down on the keys of your piano and you just played a jazz chord.

Any geek should love music theory. Especially when you realize that music is much like computer programming: it's beautifully logical, and yet we're still much better at it than any computer. :-)
People seem to be interpreting the whimsical tone of my writeup to mean that I don't know what I'm talking about. Rest assured that I mostly do. :-)

Art Tatum: I don't really understand the point of your analogy. Different shades of the same color don't really compare to octaves--color shades are only a result of varying intensity of the same hue, whereas octaves are a result of two pitches whose frequencies share a certain property (they are in the ratio of 1:2^n). If the color spectrum were periodic in such a way that every period had a color that was like a color in the last period, yet somehow different, then your analogy would apply.

srkorn: Yes, I understand the idea of a fundamental frequency and resulting overtones, but that still does nothing to explain the psychoacoustic reasoning behind human perception of the sound of an octave. How would you explain to a deaf person what an octave sounds like? What could you say, besides "They're different, but somehow they sound the same?"